The present invention relates generally to spacecraft communication systems and methods, and more particularly, to a single-receiver, multiple-antenna RF autotrack control system and method for precision pointing of multiple antennas to compensate for disturbances, such as those experienced by a spacecraft.
The assignee of the present invention manufactures and deploys communication satellites or spacecraft that use multiple communication antennas. RF autotracking systems have heretofore generally been used to steer individual antennas to compensate for disturbances to the spacecraft. Typically, only single control algorithms were used to steer an antenna. Sensing and actuation occurred at a high rate to compensate for the fastest disturbance, such as spacecraft motion, which typically affect all the antennas on the spacecraft in the same manner.
In certain prior systems that provide for RF autotrack control of multiple antennas, each antenna utilized a single control algorithm and a dedicated set of sensors to compensate for the most rapid disturbance. As the number of antennas increases, a dedicated sensing system for each antenna becomes prohibitive due to cost and mass.
To overcome the limitations of the above-mentioned prior art systems, U.S. Pat. No. 5,940,034 assigned to the assignee of the present invention discloses an autotrack control scheme that uses two or more receivers to provide RF autotrack control of multiple antennas. This control scheme uses two control algorithms and sums the result for each antenna that is tracked. One control algorithm corrects for rapid, common mode disturbances such as spacecraft motion disturbances, while the other control algorithm corrects for individual disturbances, such as thermal distortion, that do not affect all antennas in the same manner. The present invention provides for an improvement over the teachings of U.S. Pat. No. 5,940,034.
Thus, the most relevant prior art method for autotracking multiple antennas requires a separate tracking receiver for each autotracked antenna. The teachings of U.S. Pat. No. 5,940,034 disclose a method for reducing the amount of hardware by using two receivers, one of which is connected to a dedicated antenna while the other is switched between the remaining antennas (referred to as the multiplexed antenna). This allows the dedicated antenna to sense common disturbances while unique disturbances, such as thermal distortion, are sensed by the individual antennas.
It would therefore be desirable to have an autotrack control system for use with multiple antennas that has reduced hardware requirement. Accordingly, it is an objective of the present invention to expand upon and generalize the prior art concepts, enabling implementations with significantly less hardware. Taken to the logical conclusion, these concepts provide for a single-receiver, multiple-antenna RF autotrack control system and method for autotracking multiple antennas to compensate for disturbances, such as those experienced by a spacecraft.
To accomplish the above and other objectives, the present invention further reduces the hardware requirements compared to the teachings of U.S. Pat. No. 5,940,034. This is achieved by using frequency domain concepts to develop new insight into the problem and new techniques for solving the problem.
An exemplary embodiment of the RF autotrack control system comprises a plurality of RF feeds or feed arrays (sensors) coupled by way of an input multiplexer to a pseudo-monopulse coupler and tracking receiver. The plurality of sensors collect or sense RF energy derived from a plurality of antennas. The output of the receiver is input to a high pass filter and to an output multiplexer.
Outputs of the output multiplexer are respectively coupled to a plurality of low pass filters associated with a corresponding plurality of antennas. Outputs of the plurality of low pass filters are input to first inputs of a plurality of summing devices. The output of the high pass filter is input to second inputs of the plurality of summing devices. The outputs of the summing devices comprise a plurality of net error signals that are coupled to a plurality of control algorithms and antenna steering mechanisms associated with the plurality of antennas.
The pseudo-monopulse coupler and tracking receiver outputs azimuth and elevation error signals associated with the antennas. The output of the high pass filter comprises the high frequency portion of this error signal detected by the sensor. This high frequency information is assumed to be common for all controlled antennas (i.e., represents spacecraft motion).
The concept implemented by the present invention thus estimates the high frequency (or xe2x80x9cfastxe2x80x9d) errors using the currently selected antenna and a high-pass filter. Alternatively, the high frequency errors may be estimated using a sensor mounted on the spacecraft, such as a gyro or star tracker. Alternatively, the high frequency errors may be estimated using information from the spacecraft bus attitude control system such as planned thruster firings, for example. Alternatively, the high frequency information may be estimated using any combination of data from the above sources. The low frequency (or xe2x80x9cslowxe2x80x9d) errors are estimated using measurements from each selected antenna.
The high frequency signal is combined with the low frequency signal for each antenna, and the resulting full spectrum signal is used by a control algorithm to command each antenna pointing mechanism. Thus, the algorithm implemented in the present invention explicitly accounts for the frequency content of each disturbance source.
Unlike the teachings of U.S. Pat. No. 5,940,034, which requires simultaneous sampling from two or more antennas, the present invention only requires sampling from one antenna at any one time, reducing the necessary hardware to only one RF receiver. The present invention generalizes and expands upon the prior art by using a single controller with two signal paths, the high frequency path, representing disturbances that are common to all of the antennas, and a low frequency signal, which enables the controller to estimate the orientation of each individual antenna relative to the high frequency sensor. The two signal paths are combined using the concept of complimentary filter design.
The resulting system can use any source for the high frequency signal including an inertial measurement unit (or gyro), a second RF sensor (as in the prior art), or the currently selected antenna. In addition, information from the spacecraft body attitude control system can be used to augment the sensed high frequency signal, enabling more precise control in the case of a single receiver implementation.
It is to be noted that, by changing the source for the high frequency data to a dedicated antenna, the present generalized algorithm may be reduced to the prior art implementation as a special case.
Although the present invention reduces the required hardware, it requires more complexity in the control system design and control system software. The teachings of U.S. Pat. No. 5,940,034 provide for simple switching between the available signals, while the present invention combines and filters the signals. The design of these filters requires significant frequency-domain knowledge, which complicates the design process.
The present invention enables precision pointing of multiple antennas while requiring only one RF receiver. The present invention has lower cost, less mass, less on-board hardware, and better reliability compared to other prior approaches.
An exemplary embodiment of the RF autotrack control method involves the following steps. RF energy derived from a plurality of antennas is sensed by a plurality of RF sensors. The sensed RF energy is processed to generate pointing error signals (azimuth and elevation pointing errors). The pointing error signals are filtered by a high pass filter, and are also filtered by a plurality of low pass filters associated with each of the plurality of antennas. The respective filtered signals are combined to generate a plurality of net error control signals comprising compensation signals or commanded steps. The net error control signals or command steps are applied to the selected antenna to correct pointing errors associated with the selected antenna.